1. Field of Invention
The present invention relates to a method for producing an Fe--Ni alloy sheet and an Fe--Ni alloy shadow mask which is formed by means of etching an Fe--Ni alloy sheet. More particularly, the present invention provides an improvement in the production method of an Fe--Ni alloy sheet so as to suppress the formation of streaks which appear when apertures are pierced for passing an electron beam through the shadow-mask material.
2. Description or Related Art
Heretofore, mild steel has been ordinarily used as the material of the shadow mask in a color Braun tube. The shadow mask undergoes temperature-rise, and hence thermal expansion, due to the irradiation of the electron beam during continuous operation of the color Braun tube. This thermal expansion causes shift between the irradiating position of an electron beam and fluorescent material and hence color deviation on the color Braun tube. Therefore, an Fe--Ni alloy having a low coefficient of thermal expansion, which is referred to as the "36 alloy", is used to prevent the color deviation.
A shadow mask is formed by etching an Fe--Ni alloy sheet to pierce an aperture for passing an electron beam therethrough. When a light source is then located at the rear side of a mask and the mask is observed at the front side, a streak-form pattern elongating in the rolling direction is occasionally observed. This is a defect referred to as streak failure.
A failure in the form of streaks is occasionally formed in the case of the shadow mask made of soft steel. It is known that this is mainly due to non-metallic inclusions and carbides. The streaks are not prevented in the case of an Fe--Ni alloy, even by means of decreasing the non-metallic inclusions. The streaks of an Fe--Ni alloy are believed to be attributable to the composition segregation which is inherent in binary alloys, of which the Fe--Ni alloy is one.
There are proposals for decreasing the composition segregation of an Fe--Ni alloy as follows. (1) Japanese Unexamined Patent Publication No. 60-128,253, which proposes to heat the cast ingot, prior to forging, to a temperature of not lower than 850.degree. C. and lower than the melting point. (2) Japanese Unexamined Patent Publication No. 60-56,053 which proposes to subject the hot-rolled plate to soaking heat-treatment. (3) Japanese Unexamined Patent Publication No. 2-170,922 which proposes to subject the continuously cast slab to soaking at a temperature of from 1200.degree. to 1350.degree. C. for 1 hour or longer, heating at a temperature of from 1100.degree. to 1200.degree. C. in an atmosphere having oxygen concentration of 0.1 volume % or less and then hot-rolling.
When an Fe--Ni alloy is heated to a high temperature for a long time in air, its grain boundaries are severely oxidized. Since the heating of the cast products according to the methods of prior art (1) and (3) intends to decrease the segregation of components by means of prolonged soaking, the cast products must be subsequently ground thoroughly so as to completely remove the oxidized surface where the grain-boundary oxidation occurs. This leads to decrease the yield of product.
When a hot-rolled plate is soaked according to the prior art (2), the proportion of the grain-boundary oxidized surface layer to the plate thickness is so increased as to seriously decrease the yield of product.
In addition, although an improvement is attained by the methods of prior art (1) and (3), these methods cannot satisfactorily prevent fine streaks, which do not appear in the conventional mask but appear in the recent, very fine mask having fine pitches of apertures. A further improvement is therefore expected.